1. Field of the Invention
The present invention relates to an optical element, a lithographic apparatus including such optical element, a device manufacturing method and a device manufactured thereby.
2. Description of the Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a target portion of a substrate. Lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that circumstance, a patterning device, such as a mask, may be used to generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g. including part of one or several dies) on a substrate (e.g. a silicon wafer) that has a layer of radiation-sensitive material (resist). In general, a single substrate will contain a network of adjacent target portions that are successively exposed. Known lithographic apparatus include steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at once, and scanners, in which each target portion is irradiated by scanning the pattern through the beam of radiation in a given direction (the “scanning” direction) while synchronously scanning the substrate parallel or anti-parallel to this direction.
In a lithographic apparatus the size of features that can be imaged onto the substrate is limited by the wavelength of the projection radiation. To produce integrated circuits with a higher density of devices, and hence higher operating speeds, it is desirable to be able to image smaller features. While most current lithographic projection apparatus employ ultraviolet light generated by mercury lamps or excimer lasers, it has been proposed to use shorter wavelength radiation, e.g. of around 13 nm. Such radiation is termed extreme ultraviolet (EUV) or soft x-ray, and possible sources include, for example, laser-produced plasma sources, discharge plasma sources, or synchrotron radiation from electron storage rings.
Some EUV sources, especially plasma sources, emit radiation over a wide range of frequencies, even including infrared (IR), visible, ultraviolet (UV) and deep ultraviolet (DUV). These unwanted frequencies will propagate and cause heating problems in the illumination and projection systems and cause unwanted exposure of the resist if not blocked; although the multilayer mirrors of the illumination and projection systems are designed for reflection of the desired wavelength, e.g. 13 nm, they are optically flat and have quite high reflectivities at IR, visible and UV wavelengths. It is therefore necessary to select from the source a relatively narrow band of frequencies for the beam of radiation. Even where the source has a relatively narrow emission line, it is desirable to reject radiation out of that line, especially at longer wavelengths. It has been proposed to use a thin membrane as a filter to perform this function. However, such a film is very delicate and becomes very hot, 200-300° C. or more, leading to high thermal stresses and cracking, sublimation and oxidation in the high power levels necessary in a lithographic projection apparatus. A membrane filter also generally absorbs at least 50% of the desired radiation.
U.S. Pat. No. 6,678,037 describes a lithographic projection apparatus wherein a grating spectral filter is used in the radiation system of the lithographic projection apparatus. This grating spectral filter is designed for passing radiation of desired wavelengths to form a beam of radiation and for deflecting radiation of undesired wavelengths. The grating spectral filter is substantially formed of a material having a complex refractive index close to unity at the desired wavelengths and includes silicon protrusions (this structure is ‘invisible’ for the EUV radiation). The protrusions have a laminar sawtooth profile or a laminar square wave profile (FIGS. 3 and 4 of U.S. Pat. No. 6,678,037, respectively). Further, the structures might have a Ru coating creating an rms surface roughness of 1 nm.
A problem with optical filters with such coatings is that they also reflect a large amount of desired radiation, whereas transmission (through the protrusions) is required.